This invention relates to electrohydraulic control systems for positioning valves admitting steam to a steam turbine and more particularly to controlled-temperature transfers between full arc and partial arc modes of valve operation in a steam turbine.
The principles of operating steam turbines in the full arc mode and the partial arc mode are well known. A typical steam turbine in a turbine-generator unit of an electric power plant includes a number of steam admission arcs spaced about the circumference of the turbine casing and a number of control valves through which steam flows into the arcs and then into the turbine. When changes in load or flow are accommodated by simultaneously opening or closing all control valves, the turbine is said to be operating in the full arc mode. (In some turbines full arc mode operation involves setting all control valves wide open, then accommodating load changes by opening and closing a stop valve upstream of, and in series with, the control valves.) When, on the other hand, the control valves are opened or closed in a prescribed sequence to accommodate changes in turbine load or flow, thus admitting steam at different flow rates to different portions of the turbine circumference, the turbine is operating in the partial arc mode. Generally, operation in the partial arc mode is desirable at certain steady partial load conditions since lower throttling losses and better heat rates can be achieved than with full arc operation, while full arc operation is preferred during startup of the turbine since it permits temperature increases of the turbine inlet and first stage to occur move evenly about the turbine circumference, thus yielding lower stresses than would result from partial arc operation. The full arc mode may also be useful as an intermediate operating condition during a scheduled large load increase between two steady partial arc operating modes to limit the stresses of components such as the turbine rotor or casings or to permit increased loading rates.
A number of prior art valve control systems describe means to transfer between modes to utilize the respective advantages of the full arc and partial arc modes, and some known systems disclose features to avoid thermal shocks or the need for load level adjustments during transfer such as by attempting to keep the total steam flow rate constant during transfer. For example, U.S. Pat. No. 3,981,608 to Sato et al discloses an electrohydraulic control system wherein constant flow rate full arc-to-partial arc transfers are achieved by closing a first valve to its partial arc position while biasing the remaining valves open at a rate to maintain a constant total flow, then holding the first valve position constant while repeating the technique with successive valves until all valves are in their partial arc positions. U.S. Pat. No. 3,403,892 to Eggenberger et al, assigned to the assignee of the present invention and whose disclosure is incorporated herein by reference thereto, describes an electrohydraulic system for controlling steam valves which effects a mode transfer while attempting to maintain substantially constant turbine steam flow by simultaneously adjusting the gains and biases of electrohydraulic amplifiers which position the valves. U.S. Pat. Nos. 3,637,319 and 3,740,588 to Stratton et al, both assigned to the present assignee and whose disclosures are also incorporated herein by reference thereto, describe respectively a method and apparatus wherein a pulse generator or time ratio switching circuit is used instead of the potentiometers of U.S. Pat. No. 3,403,892 to vary amplifier biases and gains to achieve a smooth mode transfer. And U.S. Pat. No. 3,956,897 to Zitelli et al discloses a digital transfer control system wherein gradual mode transfers are effected by applying frequency-modulated pulses to a valve control mechanism.
The foregoing systems may help avoid thermal shocks to certain steam turbine components by permitting valve mode transfers to occur gradually, and may, by maintaining steam flow approximately constant during a mode transfer, limit the total temperature change associated with a transfer. However, none of the above-cited patents suggest means for controlling the rate of change of turbine temperature during a mode transfer, which would permit better management of turbine stresses and also allow combined or coordinated loading changes and mode transfers, and hence faster turbine startups and shutdowns at desirably low stresses.
Although it has been suggested in a thesis submitted to Polytechnic Institute of Brooklyn in 1970 ("Admission Control of Steam Turbines" by Mr. R. J. Dickenson) that steam flow could be held constant and first stage turbine temperature caused to vary linearly during a mode transfer, the system proposed therein to accomplish this transfer is complex and impractical with analog circuitry because of the many non-linear correction functions required.
Accordingly, it is a general object of the invention to provide a steam turbine electrohydraulic control system which permits controlled-temperature transfers between two modes of valve operation.
It is another object of the invention to provide an improved, simple electrohydraulic control system for effecting a transfer between the full arc and partial arc modes of valve operation such that total steam flow remains substantially constant and first stage turbine casing temperature varies substantially linearly with an admission reference factor indicative of the valve mode.
It is a further object of the invention to provide a method of transferring between two modes of operation of steam turbine valves whereby total steam flow is held substantially constant and first stage turbine temperature varies substantially linearly with admission reference factor.